The programmed control of viral gene expression has often served as a simplified model of the processes occurring in cells. Bacterial viruses have been particularly useful in this respect, and have generated both concepts and techniques that have aided investigations of regulatory mechanisms in eukaryotic cells. Due to its well characterized genetics and ease of handling, bacteriophage T7 offers an attractive system in which to study the regulation of viral gene expression, and the control of host gene expression after virus infection. We intend to examine the mechanism by which this virus inactivates the host RNA polymerase after infection, and to define the kinetics of synthesis of the late classes of phage messenger RNA. Using techniques that we have developed in this laboratory to study the transcription of bacteriophage T7, we will analyze the RNA synthesized by subviral particles of vaccinia virus in vitro, and compare them to products made early after infection of human cells with this virus. Such comparative studies may allow us to uncover regulatory mechanisms operative in this virus.